JB bore paste is nothing better than toothpaste. Get a .520 ball and try that instead of screwing with the bore the barrel maker made.

When you want to lap get Clover compound kit and start in the small grit but use a poured lead lap. A mop on a brush don't cut it for lapping.

Pay the money to the barrel maker to hand lap it before you ruin the bore and blame the maker.

Just MHO